A. E. Berkowitz

Exchange anisotropy * a review

Abstract Exchange anisotropy refers to the magnetic manifestations of an exchange coupling at the interface between two different magnetically ordered systems. Of particular current technological interest is the unidirectional anisotropy, or `exchange-bias’ field produced in a ferromagnetic film that is coupled to an appropriate antiferromagnetic film. Experimental characterization and theoretical models are discussed for these types of bilayers for a variety of metallic and oxide film couples.

Giant magnetic field dependent impedance of amorphous FeCoSiB wire

The impedance of an amorphous Fe4.3Co68.2Si12.5B15 wire (100 μm diameter) exhibits an extraordinarily large frequency dependent resistance in addition to the previously reported frequency dependent wire reactance. The frequency response of both the resistance and reactance is almost entirely suppressed by an axial magnetic field HA<150 Oe, resulting in a typical magnetoresistance for frequencies f<1 MHz of the order of the dc wire resistance. The magnetoresistance at f=1 MHz is ΔR/Rsat=370%. As the bulk of the magnetic response occurs for HA<5 Oe, this system shows great technological promise. We give a quantitative analysis of the phenomenon, which is rooted in classical electrodynamics.

Influence of Crystallite Size on the Magnetic Properties of Acicular γ‐Fe2O3 Particles

Magnetic properties have been measured for a number of samples of acicular γ‐Fe2O3 particles of the type used in magnetic recording tapes. The average particle size and shape were approximately the same for all samples. However, the average crystallite sizes of the samples, as determined from x‐ray line broadening, ranged from 50 to 700 A. All magnetic properties measured showed a strong dependence on average crystallite size. Saturation magnetization at room temperature, σ, decreased sharply with decreasing crystallite size. An excellent fit to the σ vs crystallite size data was made by assuming that the crystallites were separated by a nonmagnetic grain boundary on the order of 6 A wide. Room‐temperature coercive force decreased with decreasing crystallite size, and the ratio Hc(83°K)/Hc(293°K) increased sharply with decreasing crystallite size.The data support a particle model in which the constituent crystallites interact magnetostatically across nonmagnetic grain boundaries. A critical crystallite di...

Magnetoresistance of magnetite

The magnetoresistance behavior of Fe3O4 in polycrystalline thin film, powder compact, and single-crystal form are compared. Negative magnetoresistance with peaks at the coercive field, observed in thin films and powder compacts but not in the single crystal, is due to field-induced alignment of the magnetization of contiguous grains. The effect is associated with intergranular transport of spin-polarized electrons.

Magnetic anomalies in NiO nanoparticles

As first noted by Neel, antiferromagnetic nanoparticles could exhibit superparamagnetic relaxation of their spin lattices as well as permanent moments arising from uncompensated surface spins. Several samples of antiferromagnetic NiO nanoparticles with average sizes ranging from 50 to >800 A were investigated in the present study. In addition to the inverse dependence on average particle size of the susceptibility predicted by Neel, and previously reported, some unusual behavior was observed. Above the blocking temperatures (TB) of the particles, the reversible magnetization could not be fit with a Langevin function that was consistent with the physically reasonable moment representing the uncompensated spins. For the 53 A diameter particles, both zero-field-cooled (ZFC) and field-cooled (FC) loops below TB exhibit large coercive forces (several kOe) and the loops showed irreversibility up to 50 kOe. In addition, in the FC state below TB the hysteresis loops were strongly shifted. The latter behavior may ...

Oxygen as a surfactant in the growth of giant magnetoresistance spin valves

We have found a novel method for increasing the giant magnetoresistance (GMR) of Co/Cu spin valves with the use of oxygen. Surprisingly, spin valves with the largest GMR are not produced in the best vacuum. Introducing 5×10−9 Torr (7×10−7 Pa) into our ultrahigh vacuum deposition chamber during spin-valve growth increases the GMR, decreases the ferromagnetic coupling between magnetic layers, and decreases the sheet resistance of the spin valves. It appears that the oxygen may act as a surfactant during film growth to suppress defects and to create a surface which scatters electrons more specularly. Using this technique, bottom spin valves and symmetric spin valves with GMR values of 19.0% and 24.8%, respectively, have been produced. These are the largest values ever reported for such structures.

Surface spin disorder in ferrite nanoparticles (invited)

Anomalous magnetic properties of organic coated NiFe2O4 nanoparticles have been reported previously (Berkowitz et al.).5 These properties included low magnetization with a large differential susceptibility at high fields and shifted hysteresis loops after field cooling, while Mossbauer spectra indicated that all of the material was magnetically ordered. In the present study, we find that the lack of saturation in high fields is accompanied by irreversibility (i.e., hysteresis loops are open) up to 160 kOe. In addition, the particles exhibit time dependent magnetization in 70 kOe applied field. The high field irreversibility and the loop shift both vanish above 50 K. We propose a model of the magnetization within these particles consisting of ferrimagnetically aligned core spins and a spin- glass-like surface layer. We find that qualitative features of this model are reproduced by a numerical calculation of the spin distribution. The implications of this model for possible macroscopic quantum tunneling in ...

Sensitive field‐ and frequency‐dependent impedance spectra of amorphous FeCoSiB wire and ribbon (invited)

Conflicting reports of large magnetoresistive and magnetoinductive effects in amorphous FeCoSiB wires and ribbons prompted the impedance measurements reported here. The spectra (0≤f≤3.2 MHz) were obtained at room temperature using a commercial impedance analyzer both as functions of axial magnetic field (−140<HA<140 Oe) and sense current (1≤Irms≤60 mA). The phase shift due to the test leads was carefully measured and subtracted from the raw data to resolve the spectra into resistive R(f ) and reactive X(f ) components. We find for the Fe4.3Co68.2Si12.5B15 wire (120 μm diameter) and ribbon (20 μm thick) that both R(f ) and X(f ) depend strongly on frequency and magnetic field. For HA=0, each component increases monotonically with frequency, with R(f=0)≊1 Ω/cm and X(f=0)=0. In high fields (HA=140 Oe), R(f ) and X(f ) are nearly frequency independent. The field‐dependent response is sharply peaked about HA=0; the full width at half maximum is FWHM≤20 Oe, typically. The change in R(f ) and X(f ) between these...

Exchange anisotropy in coupled films of Ni81Fe19 with NiO and CoxNi1−xO

Shifted hysteresis loops were used to investigate exchange anisotropy in 500 A CoxNi1−xO/300 A Ni81Fe19 polycrystalline bilayer couples. Bilayers of Ni81Fe19 with NiO have a room‐temperature exchange field, He, of 30 Oe in the as‐deposited state. A maximum in the exchange field at room temperature was observed near x=0.4, indicating an optimal alloying of the properties of the high anisotropy CoO and the high Neel temperature NiO. The blocking temperatures of the exchange couples vary linearly with x, suggesting a linear dependence of the oxide Neel temperature with x.

Magneto‐impedance effect in NiFe plated wire

The voltage V(t) induced by a sinusoidal drive current traversing a 125‐μm‐diameter BeCu wire plated with 1 μm NiFe is observed to be a very strong function of axial dc magnetic field. The basic physics of this phenomenon is explained classically in terms of Faraday’s law of induction and Stoner–Wohlfarth magnetization reversal, which yield theoretical predictions that are in good agreement with experiment. For drive current amplitudes of the order of 100 mA, and frequencies of the order of 5 MHz, the field sensitivity dV/dH can be as large as 1 V/Oe (per cm of wire), which offers the potential for application in relatively simple, very high sensitivity magnetic field sensors.